Activated cathodes for electron



Jan. 31, 1956 v 1 qs 2,733,173

ACTIVATED CATHODES FOR ELECTRON DISCHARGE DEVICE Filed July 6, 1951 V lnven lror:

Vici'or JFrancis, b 5 W0 A His 'At t'orneq.

United States Patent ACTIVATED CATHODES FOR ELECTRON DISCHARGE DEVICE Victor James Francis, North Wembley, England, assignor to General Electric Company, a corporation of New York Application July 6,1951, Serial No. 235,431 Claims priority, application Great Britain July 13, 1950 3 Claims. or. 117-219 This invention relates to electric discharge lamps of the type comprising within a sealed envelope a gas or vapor filling and electrodes for the passage of a discharge through said filling, wherein each electrode which acts as a cathode during normal operation of the lamp is activated by the provision, at or near its surface on which the discharge terminates in normal operation, with a store of electron emissive material arranged to be heated at least partly by the discharge so as to emit thermionically in normal operation of the lamp. In the case of a lamp for A. C. operation, each electrode acts as a cathode during normal operation and is activated by the provision of a store of electron emissive material.

The main purpose of the electron emissive material is to reduce the cathode fall in potential along the discharge so as to enable the lamp to start and operate at voltages much lower than would otherwise be possible, and to prevent electrode sputtering, and for this purpose it is desirable that the material should readily liberate a copious supply of electrons when heated. However, the more active materials are readily evaporated and since the life of a lamp of the type specified is usually limited by emission failure of one of the electrodes, it is desirable that the material should not be all evaporated for a suitably long time. Accordingly, the most active material is not always the most suitable for a particular kind of lamp, and there are other factors affecting the choice of material, such as the ability to withstand disruption by the discharge.

It has, therefore, become usual to use in some lamps required to pass a relatively large current in normal operation, for example high pressure metal vapor discharge lamps, a form of material which is less active than that used in lamps required to pass relatively small currents, for example the well known low pressure tubular fluorescent lamps. In the first mentioned kind of lamp the electron emissive material is sometimes provided in the form of a relatively hard rod, for example of sintered alkaline earth oxides or thorates, enclosed by a spiral of tungsten wire, while in the other kind of lamps the material usually consists of a relatively soft layer of material, such as alkaline earth oxides formed from a suspension coated or sprayed onto a spiral of tungsten wire. Relatively hard forms of coating, such as barium oxide formed from barium hydroxide, are also known and may be used, for example, in sodium vapor lamps.

Such relatively hard material is less active initially than the softer form of material, but its activity appears to increase during the initial period, for example extending over about 50 to 100 hours, of use of the lamp until a stable condition is reached in which the material maintains a good emissivity, probably as good as that of the initially more active material, for a relatively long period, which is usually some thousands of hours. However, I have found that during the initial period of running of the lamp starting of the discharge may be unreliable and there may be undesirable sputtering of metal from the electrodes of the lamp because of unstable or inadequate 2,733,17 Patented Jan. 31, 1956 emission of electrons from the activating material, and the main object of this invention is to overcome this difliculty.

I have found that this object can be achieved by including in the store of electron emissive material for each cathode electrode both a quantity of the initially less active material and a quantity of the initially more active material arranged so that both can produce activation of the electrode.

The more active material than ensures adequate activation of the electrode during the initial period of use of the lamp so that a suitably low starting voltage can be reliably attained and sputtering avoided, and during this 7 for increasing the life of the cathode electrodes, since the activity but has a relatively short and strontium oxides, resulting from the less active kind of material, which would not initially be suitable by itself, will have conditioned itself to a suitable activity by the time the more active kind of material has become exhausted, and the lamp can then continue to run with the less active kind of material alone.

According to the invention, therefore, in a gaseous electric discharge device, the store of electron emissive material for each said cathode electrode includes both a quantity of material which has initially a relatively high life and a quantity of material which has initially a relatively low activity but has a relatively long life and whose activity increases during the initial period of use of the lamp, both of said materials being arranged to produce activation of the electrode.

The material of relatively low activity may, for example, be provided in the form of a relatively hard rod enclosed within a spiral of refractory wire, such as tungsten, and the more active material be provided in the form of a relatively soft coating either on the rod or on the wire; alternatively, for example, the material of relatively low activity may be provided in the form of a relatively thick and hard coating on a wire electrode and the more active material be lightly applied as a further thin coating on the electrode.

One embodiment of the invention will now be described by way of example in connection with the drawing which is an elevation of a lamp containing electrodes activated in accordance with the invention.

In this embodiment the lamp is a sodium vapor discharge lamp adapted to dissipate Watts in A. C. operation. Apart from the active material provided on the electrodes, the lamp is of well-known construction, having a U-shaped glass envelope 1 supported within a glass outer jacket (not shown) and containing a filling of sodium together with neon for facilitating starting of the discharge.

The thermionic electrodes 2, 2 are mounted within the envelope one at each end of a limb of the U in the usual manner, and each consists of a length of coiled-c0il tungsten wire wound into a double helix; the helix is coated, and the interstices of the coiled-coil wire filled,

' with a relatively thick covering of alkaline earth oxide,

for example barium oxide, in a dense form resulting from decomposition of a barium hydroxide coating. Over this relatively thick covering is applied a relatively thin coating of alkaline earth oxide, for example barium decomposition of the carbonates applied by spraying the electrode with a suspension of the carbonates in a solution of cellulosic binder such as nitrocellulose in butyl acetate.

The following is a further detailed description of one form of cathode and a method of manufacture thereof. A primary coil is formed by winding a tungsten wire of 0.05 mm. diameter on a molybdenum mandrel of 0.15 mm. diameter at a pitch of 200 per cent. This primary coil is Wound into a secondary coil by coiling it around a tungsten mandrel of 0.25 mm. diameter at a pitch of 150 per cent. The primary and secondary mandrels are retained after coiling. The secondary coil is then formed into a tertiary coil in the shape of a double helix as shown in the drawing. The helix may have a diameter of 5.5 mm. and a pitch of 3.5 mm., the length of the helix being 7.5 mm.

After coiling, the electrode is first provided with a layer of less active coating by dipping the electrode in a coating suspension comprising 108 gm. barium carbonate, 120 gm. strontium carbonate, and 225 cc. distilled water. The coating is allowed to dry in air on the electrode, and further coatings are then applied, if necessary, until the total weight of the coating on the electrode'is 20 mgm.

The electrode is then placed in an atmosphere consisting of four parts of carbon dioxide and one part of hydrogen, from which all oxygen has been removed, and which has been bubbled through water. An electric current of approximately 5 amperes is then passed through the electrode so that it is heated to a temperature of approximately 1000 C. The carbon dioxide ensures that the bulk of the coating remains as carbonates, the presence of the hydrogen preventing oxidation of the electrode. The presence of water vapor causes a quantity of barium hydroxide to be formed which fuses and thus fluxes the carbonate to a hard coherent mass.

For forming the more active coating, the coated electrode is sprayed with a suspension comprising 1500 gm. barium carbonate, 1500 gm. strontium carbonate, 2700 cc. of 3 per cent nitrocellulose in butyl acetate, and 1800 cc. methyl alcohol. The suspension may be milled for 24 hours. After spraying, a current of approximately 2 amperes is passed through the electrode, in air, so that the electrode is heated to approximately 500 C.

The electrode coating is finally formed (that is, changed from carbonate to oxide) during the processing of the lamp, by heating the electrode in vacuo after the electrode has been sealed into the lamp. The total weight of the emissive coating should then be about 30 mgm.

It will be understood that the invention is not to be limited to the details set forth above butthat various changes and modifications may be made within the spirit of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of activating a cathode for electron discharge devices which comprises forming on a base metal cathode member a layer of electron emissive mate'- rial comprising a quantity of alkaline earth metal hydroxide, and then applying over said layer a coating of a suspension of alkaline earth metal carbonates and subsequently heating the cathode in vacuo to decompose the said carbonates to oxides.

2. The method of activating a cathode for electron discharge devices which comprises applying to a base metal cathode member a layer of an aqueous suspension of alkaline-earth metal carbonates, heating the cathode member in a moist atmosphere of carbon dioxide and hydrogen to a temperature of the order of 1000 C. for a time sutficient to cause a quantity of alkaline-earth metal hydroxide to be formed and fused to flux the carbonates to a hard coherent mass, applying over said layer a coating of a suspension of alkaline-earth metal carbonates in a cellulosic binder, and subsequently heating the cathode member in vacuo to decompose the carbonates to oxides.

3. The method of activating a cathode for electron discharge devices which comprises applying to a base metal cathode member a layer of an aqueous suspension of alkaline-earth metal carbonates, heating the cathode member in a moist atmosphere of carbon dioxide and hydrogen to a temperature of the order of 1000 C. for a time sufiicient to cause a quantity of alkaline-earth metal hydroxide to be formed and fused to flux the carbonates to a hard coherent mass, applying over said layer a coating of a suspension of alkaline-earth metal carbonates in a cellulosic binder, heating the cathode member in air to a temperature of approximately 500 C. to decompose the cellulosic binder, and subsequently heating the cathode member in vacuo to decompose the carbonates to oxides.

References Cited in the file of this patent UNITED STATES PATENTS 1,691,446 Rentschler Nov. 13, 1928 2,179,110 Widell Nov. 7, 1939 2,226,281 Ewing Dec. 24, 1940 2,249,598 Borchardt July 15, 1941 2,249,672 Spanner July 15, 1941 2,459,841 Rouse Jan. 25, 1949 OTHER REFERENCES Reich: Theory and Applications of Electron Tubes, First Edition, McGraw-Hill Book Co., Inc., New York 1939, page 19. 

1. THE METHOD OF ACTIVATING A CATHODE FOR ELECTRON DISCHARGE DEVICES WHICH COMPRISES FORMING ON A BASE METAL CATHOD MEMBER A LAYER OF ELECTRON EMISSIVE MATERIAL COMPRISING A QUANTITY OF ALKALINE EARTH METAL HYDROXIDE, AND THEN APPLYING OVER SAID LAYER A COATING OF A SUSPENSION OF ALKALINE EARTH METAL CARBONATES AND SUBSEQUENTLY HEATING THE CATHODE IN VACUO TO DECOMPOSE THE SAID CARBONATES TO OXIDES. 